Alderaan is the name of a cloud of space dust, formerly not a cloud of space dust, but that was before it Asploded in 1977 AD (0 BBY).

Lucas intended Alderaan to serve as a decoy in case some evil maniac bent on conquering the galaxy decides to destroy Earth. Indeed, when the Empire announced its Star Wars program, a counter-rocket defense program against the Soviet Union, Earth was planned to be their primary military target. Initially, Princess Leia (a shining star and deadly warrior, nicknamed Death Star), the leader of the invasion on Earth, wanted to destroy Dantooine, but the rest of the Imperial military was against that idea, since they were all secretly writing illegal KOTOR fanfics and were afraid that their predictions of Revan living happily ever after with Bastila Shan on Dantooine for 4,000 years (describing their relationships with great detail) would contradict reality.

As Dantooine was unanimously declared an unsuitable target, Leia and her father launched an attack against Earth. Unfortunately, they mistook Alderaan for Earth, as the two planets looked almost identical - which was exactly what Lucas intended. Since nobody really cared about Alderaan, nobody believed the Empire was evil.

When Earth was once again placed in jeopardy during George W. Bush's War on Terra campaign, Lucas created yet another clone of Earth, and called it Naboo. However, by doing so he accidentally spawned one of the galaxy's most sinister evils - Emperor PalpatineJar Jar Binks. Whether Lucas' attempts to save Earth will succeed is yet to be seen.